The selective role of halogen and hydrogen bond interactions in ionization processes occurring in planetary atmospheres and induced by excited species

Gas phase chemiionization processes of water, hydrogen sulfide, and ammonia molecules, induced by excited metastable noble gas atoms, known as Penning ionizations, play an important role in several phenomena occurring in low energy ionized plasmas, electric discharges, planetary atmospheres, and interstellar environments. These reactions, in particular the ionization of H2O, H2S and NH3 molecules by collision with metastable neon and helium atoms, have been studied in crossed beam experiments by mass spectrometry and by energy analysis of the electrons emitted during the ionization processes. The experimental data have been analyzed by exploiting a formulation of the potential energy surface for atom-molecule interactions, proper for the investigation of the reaction dynamics by semi-classical methods. The combined analysis of the experimental data has indicated that a strong steric effect is present in microscopic mechanism of these reactions. This effect arises from the competition of several interaction components. Specifically, the floppy nature of the excited more external electron of the metastable rare gas atom and its ionic core play a crucial role. When such atom is approaching the molecule along the lone pair direction, the floppy excited orbital is repelled backward establishing a rather strong attraction between the permanent dipole of the molecule and the ionic core of the excited atom (halogen bond like interaction). On the other side, when the excited atom is approaching the molecule along the direction of one of the bonds with one of the hydrogen atoms a weaker interaction is established (hydrogen bond like interaction). The competition between the various interaction components, together with the charge distributed on the molecules, that vary differently with the orientation, determines a strong anisotropic behavior of the reactivity.